According to "Cellular Materials," Encyclopedia of Polymer Science and Engineering, vol. 3, pages 1-59, (2d ed. John Wiley & Sons, 1985), foamed plastic is defined as a plastic in which the apparent density decreases substantially with the presence of numerous cells disposed through its mass. The gas phase in a foamed plastic is generally distributed in cells. Blowing agents produce gas used to generate cells in foamable polymeric materials, for example, to make foamed insulation. Physical blowing agents form cells by a phase change, for example, a liquid may be volatilized or a gas dissolved in a polymer under high pressure.
Commercially important liquid blowing agents are aliphatic hydrocarbons and their chloro- and fluoro-derivatives. For example, isomers of pentane, hexane, and heptane are used mainly in the production of very low density polystyrene foam. These liquids tend to be inexpensive and low in toxicity. However, they are highly flammable. See Encyclopedia, vol. 2, page 437, supra.
Methylene chloride is the most widely used chlorohydrocarbon blowing agent, it is nonflammable, and since the compound contains chlorine, methylene chloride may be subjected to the same restrictions as chlorofluorocarbons and it is, in addition, a toxic, carcinogenic compound.
As discussed in Chemical and Engineering News, Jul. 16, 1991, pages 5-6, physical blowing agents, such as low boiling liquids, particularly chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), are used throughout the world on a large scale to produce foamed plastics, for example to produce polyurethane and polyisocyanurate foams. However, CFCs are linked to the destruction of the earth's protective ozone layer. See Encyclopedia, vol 2, page 434, supra. Depletion of the ozone layer is likely to result in increased cases of skin cancer and ecosystem damage. Consequently, the major culprits are to be phased out by the year 2000, under the U.S. Clean Air Act and the Montreal Protocol. However, many environmental groups are calling for faster phaseout. Some European countries, in particular Germany, are requiring that all CFCs be replaced in polyurethane foams and polyisocyanurate foams by 1995.
Production of cellular plastic products, such as cellular polyurethane elastomers and flexible, semi-rigid or rigid polyurethane foams in the presence of catalysts, blowing agents, processing aids or additives is described in numerous patents and publications in the literature.
A survey of methods of producing cellular polyurethane elastomers, polyurethane foams and polyisocyanurate foams, their mechanical properties and their use can be found, for example, High Polymers, Vol. 14, "Polyurethanes," Parts I and II by J. H. Saunders and K. C. Frisch (Interscience Publishers, New York 1962 and 1964), Plastics Handbook, Volume VII, "Polyurethanes," 1st ed. 1966, published by R. Vieweg and A. Hochtlen and 2d ed. 1983, published by G. Oertel (Carl Hanser Verlag, Munich), and "Integral Foams," published by H. Piechota and H. Rohr (Carl Hanser Verlag, Munich, 1975).
A recent patent, U.S. Pat. No. 4,972,002 (Volkert), describes producing cellular plastics by the polyisocyanate polyaddition process by reaction of (a) organic and/or modified organic polyisocyanates with (b) at least one high molecular compound with at least two reactive hydrogen atoms and, optionally, (c) low molecular weight chain extenders and/or cross-linking agents in the presence of (d) blowing agents, (e) catalysts, (f) additives and/or processing aids, wherein the blowing agents are low boiling fluorinated aliphatic and/or cycloaliphatic hydrocarbons that have 3 to 8 carbons. For example, perfluorocyclopentane is used as a preferred blowing agent.
Another recent patent, U.S. Pat. No. 4,981,879 (Snider), describes a process for preparing cellular polymers having urethane groups, isocyanurate groups, or both. The cellular polymers are prepared by reacting an organic polyisocyanate with a polyol in the presence of a blowing agent, typically a hydrocarbon, hydrochlorofluorocarbon, or chlorofluorocarbon, a catalyst and a perfluorinated hydrocarbon or a mixture of perfluorinated hydrocarbons, such that the lower boiling perfluorinated hydrocarbons can function as a co-blowing agent.
U.S. Pat. No. 4,997,706 (Smits et al.) describes a process for preparing rigid, closed-cell, polymer foams having urethane groups, isocyanurate groups, or both, and uses C.sub.2-6 polyfluorocarbon compounds containing no chlorine or bromine atoms as a physical blowing agent.